Version 0 of stasher

Updated 2011-04-27 13:08:12 by lars_h

This page is about (the features that should be produced within) an ongoing project of the Google Summer Of Code 2011.

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The idea (abstractly): Suppose you've got some value X, and you know you'll need to know f(X) quite often, where f is some function. Then a stasher can let you compute f(X) once and then "stash" the result within the Tcl_Obj that is X, so that when subsequently you need to know f(X), computing it is "free".

The Tcl core uses this idea in a number of cases, usually when X is a piece of code written by a programmer and f is the operation of compiling that code: Tcl scripts can be bytecompiled, expressions can be bytecompiled too, and regexps compile to a finite automaton. But traditionally, you could only do that if you had implemented f in C. A stasher makes it possible to use any Tcl command as an f here, and additionally it lets you cache g(X), h(X), etc. for as many functions g, h, … as you bother to define.

Interface

(The details below are preliminary.)

Small glossary:

stash

Where cached values are stored. Technically this is going to reside within the internal representation of a Tcl_Obj, so that when the original value is forgotten, everything that was cached for it goes away too.

stasher

A command used to access a stash.

stasher

The command used to create stashers.

property

A computed value that can be cached within a stash.

(To be continued)

Example

Practically, one might do something like the following

 stasher book {
    # Lots of code defining properties of "book"s and how to compute or look them up
 }
 set pptt isbn:0-13-038560-3
 book get $pptt title

Some delay when book retrieves a library catalog entry for the book with ISBN 0-13-038560-3, and then returns: Practical programming in Tcl/Tk.

 book get $pptt authors

No delay before returning: {Welch, Brent B.} {Hobbs, Jeffrey} {Jones, Ken}, as the full catalog entry is stashed within the $pptt value.

 set pptt

Returns: isbn:0-13-038560-3. We're not automagically mutating values, we're doing things to the internal representation that supplements that value.

 book get isbn:0-13-038560-3 authors

Returns {Welch, Brent B.} {Hobbs, Jeffrey} {Jones, Ken} as above, but after a delay. Though the two strings are equal, they are not stored in the same Tcl_Obj, so when book is handed one it cannot access the data stashed within the other.

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